Human Radiation Dosimetry of [(18)F]AV-1451(T807) to Detect Tau Pathology

Human biodistribution and radiation dosimetry of two novel α-synuclein PET tracers, 18F-SPAL-T-06 and 18F-C05-05

18F-SPAL-T-06 and 18F-C05-05 are two novel positron emission tomography (PET) radioligands targeting α-synuclein fibrils. Our study aimed to evaluate the biodistribution, safety, and radiation dosimetry of each tracer in humans. Biodistribution and radiation dosimetry studies were carried out with two healthy volunteers for each tracer, 18F-SPAL-T-06 (one female and one male volunteer, both aged 63 years) and 18F-C05-05 (one female and one male volunteer, aged 63 and 73 years, respectively). After injection of either tracer, dynamic PET images were acquired from head to upper thigh. Effective dose of each tracer was estimated using OLINDA/EXM Version 2.2. Injection of either of the tracers caused no adverse effects. Greatest uptake of both tracers was observed in the liver and small intestine. The estimated absorbed doses were highest in the biliary tract, followed by the lower large intestinal wall. Effective doses were 35.9 µSv/MBq for 18F-SPAL-T-06 and 30.5 µSv/MBq for 18F-C05-05. 18F-SPAL-T-06 and 18F-C05-05 are safe for in vivo PET imaging of humans. Their mean effective doses were 6.6 mSv for 18F-SPAL-T-06 and 5.6 mSv for 18F-C05-05 when 185 MBq of either tracer was given to a subject, and they were comparable to other amyloid and tau PET tracers labelled with 18F.Trial registration Trial registration number: jRCTs031210180, Registered date: 2nd July 2021 (18F-SPAL-T-06) https://jrct.niph.go.jp/en-latest-detail/jRCTs031210180 and Trial registration number: jRCTs031220123, Registered date: 9th June 2022 (18F-C05-05) https://jrct.niph.go.jp/en-latest-detail/jRCTs031220123 .

Human biodistribution and radiation dosimetry for the tau tracer [18F]Florzolotau in healthy subjects

BACKGROUND

Tau pathology plays a crucial role in neurodegeneration diseases including Alzheimer's disease (AD) and non-AD diseases such as progressive supranuclear palsy. Tau positron emission tomography (PET) is an in-vivo and non-invasive medical imaging technique for detecting and visualizing tau deposition within a human brain. In this work, we aim to investigate the biodistribution of the dosimetry in the whole body and various organs for the [18F]Florzolotau tau-PET tracer. A total of 12 healthy controls (HCs) were enrolled at Chang Gung Memorial Hospital. All subjects were injected with approximately 379.03 ± 7.03 MBq of [18F]Florzolotau intravenously, and a whole-body PET/CT scan was performed for each subject. For image processing, the VOI for each organ was delineated manually by using the PMOD 3.7 software. Then, the time-activity curve of each organ was acquired by optimally fitting an exponential uptake and clearance model using the least squares method implemented in OLINDA/EXM 2.1 software. The absorbed dose for each target organ and the effective dose were finally calculated.

RESULTS

From the biodistribution results, the elimination of [18F]Florzolotau is observed mainly from the liver to the intestine and partially through the kidneys. The highest organ-absorbed dose occurred in the right colon wall (255.83 μSv/MBq), and then in the small intestine (218.67 μSv/MBq), gallbladder wall (151.42 μSv/MBq), left colon wall (93.31 μSv/MBq), and liver (84.15 μSv/MBq). Based on the ICRP103, the final computed effective dose was 34.9 μSv/MBq with CV of 10.07%.

CONCLUSIONS

The biodistribution study of [18F]Florzolotau demonstrated that the excretion of [18F]Florzolotau are mainly through the hepatobiliary and gastrointestinal pathways. Therefore, a routine injection of 370 MBq or 185 MBq of [18F]Florzolotau leads to an estimated effective dose of 12.92 or 6.46 mSv, and as a result, the radiation exposure to the whole-body and each organ remains within acceptable limits and adheres to established constraints.

TRIAL REGISTRATION

Retrospectively Registered at Clinicaltrials.gov (NCT03625128) on 12 July, 2018, https://clinicaltrials.gov/study/NCT03625128 .

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Radiation dosimetry and pharmacokinetics of the tau PET tracer florzolotau (18F) in healthy Japanese subjects

OBJECTIVE

Abnormal aggregation of tau in the brain is a major contributing factor in various neurodegenerative diseases. Florzolotau (18F) (florzolotau, APN-1607, PM-PBB3) has been shown to be a probe for tau fibrils in an animal model and patients with Alzheimer's disease and those with non-Alzheimer's disease tauopathies. The objective of this study is to evaluate the safety, pharmacokinetics, and radiation dose following a single intravenous administration of florzolotau in healthy Japanese subjects.

METHODS

Three healthy male Japanese subjects aged between 20 and 64 were enrolled in this study. Subjects were determined to be eligible based on the screening assessments at the study site. Subjects received a single intravenous dose of 195.0 ± 0.5 MBq of florzolotau and underwent the whole-body PET scan 10 times in total to calculate absorbed doses to major organs/tissues and effective dose. Radioactivities in whole blood and urine were also measured for pharmacokinetic evaluation. Absorbed doses to major organs/tissues and effective dose were estimated using the medical internal radiation dose (MIRD) method. Vital signs, electrocardiography (ECG), and blood tests were done for safety evaluation.

RESULTS

The intravenous injection of florzolotau was well tolerated. There were no adverse events or clinically detectable pharmacologic effects related to the tracer in any subjects. No significant changes in vital signs and ECG were observed. The highest mean initial uptake at 15 min after injection was in the liver (29.0 ± 4.0%ID), intestine (4.69 ± 1.65%ID), and brain (2.13 ± 0.18%ID). The highest absorbed dose was 508 μGy/MBq of the gallbladder wall, followed by the liver of 79.4 μGy/MBq, the pancreas of 42.5 μGy/MBq, and the upper large intestine of 34.2 μGy/MBq. The effective dose was calculated as 19.7 μSv/MBq according to the tissue weighting factor reported by ICRP-103.

CONCLUSION

Florzolotau intravenous injection was well tolerated in healthy male Japanese subjects. The effective dose was determined as 3.61 mSv when 185 MBq florzolotau was given.

Dosimetry and efficacy of a tau PET tracer [18F]MK-6240 in Japanese healthy elderly and patients with Alzheimer's disease

OBJECTIVE

A new tau PET tracer [¹⁸F]MK-6240 has been developed; however, its dosimetry and pharmacokinetics have been published only for a European population. This study investigated the safety, radiation dosimetry, pharmacokinetics and biodistribution of [¹⁸F]MK-6240 in Japanese elderly subjects. Also, the pattern and extent of brain retention of [¹⁸F]MK-6240 in Japanese healthy elderly subjects and patients with Alzheimer's disease (AD) were investigated. These Japanese results were compared with previous reports on non-Japanese.

METHODS

Three healthy elderly subjects and three AD patients were enrolled. Dynamic whole-body PET scans were acquired for up to 232 min after starting injection of [¹⁸F]MK-6240 (370.4 ± 27.0 MBq) for the former, while a dynamic brain scan was performed from 0 to 75 min post injection for the latter. For both groups, brain PET scans were conducted from 90 to 110 min post injection. Sequential venous blood sampling was performed to measure the radioactivity concentration in the whole blood and plasma as well as the percentages of parent [¹⁸F]MK-6240 and radioactive metabolites in plasma. Organ doses and effective doses were estimated using the OLINDA Ver.2 software. Standardized uptake value ratios (SUVRs) and distribution volume ratios (DVRs) by Logan reference tissue model (LRTM) were measured in eight brain regions using the cerebellar cortex as the reference. Blood tests, urine analysis, vital signs and electrocardiography were performed for safety assessments.

RESULTS

No adverse events were observed. The highest radiation doses were received by the gallbladder (257.7 ± 74.9 μGy/MBq) and the urinary bladder (127.3 ± 11.7 μGy/MBq). The effective dose was 26.8 ± 1.4 μSv/MBq. The parent form ([¹⁸F]MK-6240) was metabolized quickly and was less than 15% by 35 min post injection. While no obvious accumulation was found in the brain of healthy subjects, focal accumulation of [¹⁸F]MK-6240 was observed in the cerebral cortex of AD patients. Regional SUVRs of the focal lesions in AD patients increased gradually over time, and the difference of SUVRs between healthy subjects and AD patients became large and stable at 90 min after injection. High correlations of SUVR and DVR were observed (p < 0.01).

CONCLUSION

The findings supported safety and efficacy of [¹⁸F]MK-6240 as a tau PET tracer for Japanese populations. Even though the number of subjects was limited, the radiation dosimetry profiles, pharmacokinetics, and biodistribution of [¹⁸F]MK-6240 were consistent with those for non-Japanese populations.

TRIAL REGISTRATION

Japan Pharmaceutical Information Center ID, JapicCTI-194972.

Biodistribution and Dosimetry Evaluation for a Novel Tau Tracer [18F]-S16 in Healthy Volunteers and Its Application in Assessment of Tau Pathology in Alzheimer’s Disease

Background: The goal of this study was to report a fully automated radiosynthetic procedure of a novel tau tracer [18F]-S16 and its safety, biodistribution, and dosimetry in healthy volunteers as well as the potential utility of [18F]-S16 positron emission tomography (PET) in Alzheimer’s disease (AD).

Methods: The automated radiosynthesis of [18F]-S16 was performed on a GE Tracerlab FX2 N module. For the biodistribution and dosimetry study, healthy volunteers underwent a series of PET scans acquired at 10, 60, 120, and 240 min post-injection. The biodistribution and safety were assessed. For the AD study, both AD and healthy controls (HCs) underwent dynamic [18F]-S16 and static [18F]-FDG PET imaging. [18F]-S16 binding was assessed quantitatively using standardized uptake value ratios (SUVRs) measured at different regions of interest (ROIs). [18F]-S16 SUVRs were compared between the AD patients and HCs using the Mann–Whitney U-test. In AD patients with all cortical ROIs, Spearman rank-correlation analysis was used to calculate the voxel-wise correlations between [18F]-S16 and [18F]-FDG.

Results: The automated radiosynthesis of [18F]-S16 was finished within 45 min, with a radiochemical yield of 30 ± 5% (n = 8, non-decay-corrected). The radiochemical purity was greater than 98%, and the specific activity was calculated to be 1,047 ± 450 GBq/μmol (n = 5), and [18F]-S16 was stable in vitro. In the healthy volunteer study, no adverse effect was observed within 24 h post-injection, and no defluorination was observed in vivo. The radiotracer could pass through the blood–brain barrier easily and was rapidly cleared from the circulation and excreted through the hepatic system. The whole-body mean effective dose was 15.3 ± 0.3 μSv/MBq. In AD patients, [18F]-S16 accumulation was identified as involving the parietal, temporal, precuneus, posterior cingulate, and frontal lobes. No specific [18F]-S16 cerebral uptake was identified in HCs. The SUVR of AD patients was significantly higher than that of HCs. No specific binding uptake was found in the choroid plexus, venous sinus, and white matter. A significant correlation was found between [18F]-S16 binding and hypometabolism across neocortical regions.

Conclusion: [18F]-S16 could be synthesized automatically, and it showed favorable biodistribution and safety in humans. [18F]-S16 PET indicated a high image quality for imaging tau deposition in AD and distinguishing AD from HCs.

A Brief History of Nuclear Medicine Physics, Instrumentation, and Data Sciences in Korea

We review the history of nuclear medicine physics, instrumentation, and data sciences in Korea to commemorate the 60^th anniversary of the Korean Society of Nuclear Medicine. In the 1970s and 1980s, the development of SPECT, nuclear stethoscope, and bone densitometry systems, as well as kidney and cardiac image analysis technology, marked the beginning of nuclear medicine physics and engineering in Korea. With the introduction of PET and cyclotron in Korea in 1994, nuclear medicine imaging research was further activated. With the support of large-scale government projects, the development of gamma camera, SPECT, and PET systems was carried out. Exploiting the use of PET scanners in conjunction with cyclotrons, extensive studies on myocardial blood flow quantification and brain image analysis were also actively pursued. In 2005, Korea's first domestic cyclotron succeeded in producing radioactive isotopes, and the cyclotron was provided to six universities and university hospitals, thereby facilitating the nationwide supply of PET radiopharmaceuticals. Since the late 2000s, research on PET/MRI has been actively conducted, and the advanced research results of Korean scientists in the fields of silicon photomultiplier PET and simultaneous PET/MRI have attracted significant attention from the academic community. Currently, Korean researchers are actively involved in endeavors to solve a variety of complex problems in nuclear medicine using artificial intelligence and deep learning technologies.

Tauvid™: The First FDA-Approved PET Tracer for Imaging Tau Pathology in Alzheimer's Disease

Tauvid has been approved by the U.S. Food and Drug Administration (FDA) in 2020 for positron emission tomography (PET) imaging of adult patients with cognitive impairments undergoing evaluation for Alzheimer’s disease (AD) based on tau pathology. Abnormal aggregation of tau proteins is one of the main pathologies present in AD and is receiving increasing attention as a diagnostic and therapeutic target. In this review, we summarised the production and quality control of Tauvid, its clinical application, pharmacology and pharmacokinetics, as well as its limitation due to off-target binding. Moreover, a brief overview on the second-generation of Tau PET tracers is provided. The approval of Tauvid marks a step forward in the field of AD research and opens up opportunities for second-generation tau tracers to advance tau PET imaging in the clinic.

Preclinical Safety Evaluation and Human Dosimetry of [18F]MK-6240, a Novel PET Tracer for Imaging Neurofibrillary Tangles

PURPOSE

[¹⁸F]MK-6240 is a selective, high-affinity positron emission tomography tracer for imaging neurofibrillary tangles, a key pathological signature that correlates with cognitive decline in Alzheimer disease. This report provides safety information from preclinical toxicology studies and first-in-human whole-body biodistribution and dosimetry studies of [¹⁸F]MK-6240 for its potential application in human brain imaging studies.

PROCEDURES

MK-6240 was administered intravenously (IV) in a 7-day rat toxicity study at × 50, × 100, and × 1000 dose margins relative to projected highest clinical dose of 0.333 μg/kg. The IV formulation of MK-6240 for clinical use and the formulation used in the 7-day rat toxicity study was tested for hemolysis potential in human and Wistar rat whole blood. Sequential whole-body positron emission tomography scans were performed in three healthy young subjects after IV bolus injection of 180 ± 0.3 MBq [¹⁸F]MK-6240 to characterize organ biodistribution and estimate whole-body radiation exposure (effective dose).

RESULTS

MK-6240 administered IV in a 7-day rat toxicity study did not show any test article-related changes. The no-observed-adverse-effect level in rats was ≥ 333 μg/kg/day which provides a margin 1000-fold over an anticipated maximum clinical dose of 0.333 μg/kg. Additionally, the MK-6240 formulation was not hemolytic in human or Wistar rat blood. [¹⁸F]MK-6240 activity was widely distributed to the brain and the rest of the body, with organ absorbed doses largest for the gall bladder (202 μGy/MBq). The average (±SD) effective dose was 29.4 ± 0.6 μSv/MBq, which is in the typical range for F-18 radiolabeled ligands.

CONCLUSIONS

Microdoses of [¹⁸F]MK-6240 are safe for clinical positron emission tomography imaging studies. Single IV administration of 185 MBq (5 mCi) [¹⁸F]MK-6240 is anticipated to result in a total human effective dose of 5.4 mSv and thus allows multiple positron emission tomography scans of the same subject per year.

Use of 55 PET radiotracers under approval of a Radioactive Drug Research Committee (RDRC)

BACKGROUND

In the US, EU and elsewhere, basic clinical research studies with positron emission tomography (PET) radiotracers that are generally recognized as safe and effective (GRASE) can often be conducted under institutional approval. For example, in the United States, such research is conducted under the oversight of a Radioactive Drug Research Committee (RDRC) as long as certain requirements are met. Firstly, the research must be for basic science and cannot be intended for immediate therapeutic or diagnostic purposes, or to determine the safety and effectiveness of the PET radiotracer. Secondly, the PET radiotracer must be generally recognized as safe and effective. Specifically, the mass dose to be administered must not cause any clinically detectable pharmacological effect in humans, and the radiation dose to be administered must be the smallest dose practical to perform the study and not exceed regulatory dose limits within a 1-year period. In our experience, the main barrier to using a PET radiotracer under RDRC approval is accessing the required information about mass and radioactive dosing.

RESULTS

The University of Michigan (UM) has a long history of using PET radiotracers in clinical research studies. Herein we provide dosing information for 55 radiotracers that will enable other PET Centers to use them under the approval of their own RDRC committees.

CONCLUSIONS

The data provided herein will streamline future RDRC approval, and facilitate further basic science investigation of 55 PET radiotracers that target functionally relevant biomarkers in high impact disease states.

An one-pot two-step automated synthesis of [18F]T807 injection, its biodistribution in mice and monkeys, and a preliminary study in humans

[18F]T807 is a potent tau protein imaging agent. In order to fulfill the demand from preclinical and clinical studies, we developed an automated one-pot two-step synthesis of this potent tau imaging agent and studied its stability, and dosimetry in mice and monkeys. We also conducted a preliminary study of this imaging agent in humans. Using this one-pot two-step method, the radiochemical yield (RCY) of [18F]T807 was 20.5 ± 6.1% (n = 15) at the end of bombardment (EOB) in a synthesis time of 70±5 min. The chemical and radiochemical purities were >90% and the specific activities were 151 ± 52 GBq/μmol. The quality of [18F]T807 synthesized by this method met the U.S. Pharmacopoeia (USP) criteria. The stability test showed that the [18F]T807 injection was stable at room temperature for up to 4 h after the end of synthesis (EOS). The estimated effective dose of the [18F]T807 injection extrapolated from monkeys was 19 μSv/MBq (n = 2), while the estimated effective doses of the [18F]T807 injection extrapolated from fasted and non-fasted mice were 123 ± 27 (n = 3) and 94 ± 19 (n = 4) μSv/MBq, respectively. This one-pot two-step automated method produced the [18F]T807 injection with high reproducibility and high quality. PET imaging and radiation dosimetry evaluation in mice and Formosan rock monkeys suggested that the [18F]T807 injection synthesized by this method is suitable for use in human PET imaging studies. Thus, this method could fulfill the demand for the [18F]T807 injection in both preclinical and clinical studies of tauopathies, especially for nearby study sites without cyclotrons.

Tau-imaging in neurodegeneration

Pathological cerebral aggregations of proteins are suggested to play a crucial role in the development of neurodegenerative disorders. For example, aggregation of the protein ß-amyloid in form of extracellular amyloid-plaques as well as intraneuronal depositions of the protein tau in form of neurofibrillary tangles represent hallmarks of Alzheimer's disease (AD). Recently, novel tracers for in vivo molecular imaging of tau-aggregates in the brain have been introduced, complementing existing tracers for imaging amyloid-plaques. Available data on these novel tracers indicate that the subject of Tau-PET may be of considerable complexity. On the one hand this refers to the various forms of appearance of tau-pathology in different types of neurodegenerative disorders. On the other hand, a number of hurdles regarding validation of these tracers still need to be overcome with regard to comparability and standardization of the different tracers, observed off-target/non-specific binding and quantitative interpretation of the signal. These issues will have to be clarified before systematic clinical application of this exciting new methodological approach may become possible. Potential applications refer to early detection of neurodegeneration, differential diagnosis between tauopathies and non-tauopathies and specific patient selection and follow-up in therapy trials.

An updated radiosynthesis of [18F]AV1451 for tau PET imaging

BACKGROUND

[18F]AV1451 is a commonly used radiotracer for imaging tau deposits in Alzheimer's disease (AD) and related non-AD tauopathies. Existing radiosyntheses of [18F]AV1451 require complex purifications to provide doses suitable for use in clinical imaging studies. To address this issue, we have modified the synthesis of [18F]AV1451 to use only 0.5 mg precursor, optimized the Boc-deprotection step and developed a simplified method for HPLC purification of the radiotracer.

RESULTS

An optimized [18F]AV1451 synthesis using a TRACERLab FXFN module led to high radiochemical yield (202 ± 57 mCi per synthesis) and doses with excellent radiochemical purity (98 ± 1%) and good specific activity (2521 ± 623 Ci/mmol).

CONCLUSION

An updated and operationally simple synthesis of [18F]AV1451 has been developed that is fully automated and prepares radiotracer doses suitable for use in clinical tau PET studies.

Kinetic Modeling of the Tau PET Tracer 18F-AV-1451 in Human Healthy Volunteers and Alzheimer Disease Subjects

¹⁸F-AV-1451 is currently the most widely used of several experimental tau PET tracers. The objective of this study was to evaluate ¹⁸F-AV-1451 binding with full kinetic analysis using a metabolite-corrected arterial input function and to compare parameters derived from kinetic analysis with SUV ratio (SUVR) calculated over different imaging time intervals. Methods:¹⁸F-AV-1451 PET brain imaging was completed in 16 subjects: 4 young healthy volunteers (YHV), 4 aged healthy volunteers (AHV), and 8 Alzheimer disease (AD) subjects. Subjects were imaged for 3.5 h, with arterial blood samples obtained throughout. PET data were analyzed using plasma and reference tissue-based methods to estimate the distribution volume, binding potential (BP_ND), and SUVR. BP_ND and SUVR were calculated using the cerebellar cortex as a reference region and were compared across the different methods and across the 3 groups (YHV, AHV, and AD). Results: AD demonstrated increased ¹⁸F-AV-1451 retention compared with YHV and AHV based on both invasive and noninvasive analyses in cortical regions in which paired helical filament tau accumulation is expected in AD. A correlation of R² > 0.93 was found between BP_ND (130 min) and SUVR-1 at all time intervals. Cortical SUVR curves reached a relative plateau around 1.0-1.2 for YHV and AHV by approximately 50 min, but increased in AD by up to approximately 20% at 110-130 min and approximately 30% at 160-180 min relative to 80-100 min. Distribution volume (130 min) was lower by 30%-35% in the YHV than AHV. Conclusion: Our data suggest that although ¹⁸F-AV-1451 SUVR curves do not reach a plateau and are still increasing in AD, an SUVR calculated over an imaging window of 80-100 min (as currently used in clinical studies) provides estimates of paired helical filament tau burden in good correlation with BP_ND, whereas SUVR sensitivity to regional cerebral blood changes needs further investigation.